Page 34 - Soil Degradation, Conservation and Remediation

1.6 Classification of World Soils 19
Xerepts: Xerepts are mainly more or less freely drained Inceptisols that have a xeric

soil moisture regime. They are Inceptisols of the temperate regions with very dry
summers and moist winters. Xerepts are moderately extensive in the USA and
are the most common in California, Oregon, Washington, Idaho, and Utah. The
vegetation commonly is coniferous forest on soils with frigid or mesic tempera-
ture regimes and shrubs, grass, and widely spaced trees on the soils with a
thermic temperature regime.

1.6.8 Mollisols

Mollisols are dark-colored, base-rich mineral soils of the grasslands. They have a
mollic epipedon. They may have an argillic, natric, or calcic horizon or an albic
horizon. Some have a duripan or a petrocalcic horizon. Mollisols do not have per-
mafrost, organic soil materials, and a spodic horizon. Mollisols may have any of the
defined temperature regimes. Mollisols can have any soil moisture regime, but

enough available moisture to support perennial grasses seems to be essential.
Mollisols are used mainly for small grain in the drier regions and corn (maize) or
soybeans in the warmer, humid region. Mollisols comprise 6.9 % of the ice-free
land of the earth. (Mollisols are called Chernozems, Phaeozems, and Kastanozems
in WRB classification system.) The suborders of Mollisols are:

Albolls: Albolls are the Mollisols that have an albic horizon and fl uctuating ground-
water table. Most of these soils are saturated with water to or near the soil surface
at some time during winter or spring in normal years. These soils developed
mostly on broad, nearly level to sloping ridges, on back slopes, or in closed
depressions. Most Albolls have developed under grass or shrub vegetation.

Aquolls: Aquolls are the Mollisols that are wet and that have an aquic soil moisture

regime. In these soils, the water table remains at or near the surface for much of
the year. They have developed under grasses, sedges, and forbs, but a few have
had forest vegetation. In the USA, Aquolls are most extensive in glaciated areas
of the mid-western states where the drift was calcareous.

Cryolls: Cryolls are more or less freely drained Mollisols of the cold region. They

are abundant in the high mountains of the Western USA, on the plains and moun-
tains of Eastern Europe and in Asia. The vegetation of the Cryolls on the plains
was mostly grasses. Cryolls in the mountains have either forest or grass vegeta-
tion. Cryolls in Alaska support spruce, birch, and aspen trees.
Rendolls: Rendolls are shallow Mollisols over calcareous parent materials such as

limestone, chalk, and drift composted of limestone or shell bars of humid regions.
These soils are extensive in some parts of the world and formed under forest
vegetation or under grass and shrubs.

Udolls: Udolls are Mollisols of humid climates mainly under tall grass prairie (an
extensive, level or slightly undulating, treeless tract of land covered with coarse
grasses) vegetation, but some could have supported boreal forests (Chap. 14 )